In a hybrid electric vehicle (HEV), either or both of an internal combustion engine and an electric traction motor are capable of supplying power to the wheels of the vehicle. A traction battery is connected to the motor to provide electric power to the motor. In an electric assist mode, the engine and motor together provide power to meet driver torque demands. In an electric mode (EV-only mode), the engine is disabled while the battery and motor fulfill the driver torque demands. During the electric assist and EV-only modes, the availability of the electric power is directly tied to the available battery power.
Vehicle performance in response to aggressive acceleration requests may be degraded if the available battery power is low as typical control strategies rely on the faster response of the motor torque to satisfy such requests to meet desired energy efficiency goals. However, the torque delivered by the motor may be insufficient at low battery discharge limits and the engine may not respond quickly enough to satisfy the driver power demands. There is currently a need for a system that adequately meets driver power demands when the battery operates near low discharge limits.